1. Technical Field
The present invention relates to a temperature-compensated oscillator allowing reduction of power consumption while keeping the accurate temperature compensation, and an electronic device equipped with the temperature-compensated oscillator.
2. Related Art
In the past, a crystal oscillator such as a temperature-compensated crystal oscillator (TCXO), which is less susceptible to ambient temperature and inherent characteristics of electrical components, and is superior as a stable oscillation circuit, has been used as a reference clock source of an electronic device such as a microcomputer or a cellular phone.
FIG. 11 shows a temperature-compensated crystal oscillator described in JP-A-2003-258551 (Document 1). As shown in FIG. 11, the temperature-compensated oscillator 200 is composed of an oscillation circuit 202 and a temperature compensation circuit 206. The oscillation circuit 202 has a structure in which a plurality of series circuits each composed of a switch Sn (n denotes a natural number) and a capacitor Cn (n denotes a natural number) is connected to a circuit including a crystal resonator 204 as an oscillation source, and by setting the switches Sn to ON/OFF, it is possible to vary the capacitance inside the oscillation circuit 202 to thereby control the oscillation frequency of the oscillation signal. On the other hand, the temperature compensation circuit 206 selects a correction value for controlling the frequency so as to reduce the variation in the oscillation frequency of the crystal resonator 204 due to the temperature variation based on the temperature information obtained by a temperature sensor 208, and then outputs signals for switch control according to the correction value to the oscillation circuit 202. Further, it results that in the oscillation circuit 202, the switches S1, . . . , Sn are individually switched ON/OFF in accordance with the signals for switch control input thereto.
In the temperature-compensated crystal oscillator described in JP-A-62-38605 (Document 2), although being composed of the oscillation circuit and the temperature compensation circuit, the oscillation circuit is provided with a varactor diode with a capacity varying in accordance with the voltage applied thereto, and the temperature compensation circuit outputs a control signal for controlling the capacitance value of the varactor diode so as to reduce the frequency variation of the crystal resonator due to the temperature variation to thereby vary the frequency. Thus, the oscillation circuit applies the voltage corresponding to the control signal to the varactor diode.
Therefore, in the temperature-compensated crystal oscillator described in Document 1 or 2, it results that the capacitance inside the oscillation circuit has opposite temperature characteristics to the temperature characteristics of the oscillation frequency of the crystal resonator.
Therefore, the temperature-compensated crystal oscillator of Document 1 or 2 is capable of reducing the variation in the temperature characteristics of the oscillation frequency of the crystal resonator with the frequency variation due to the variation in the capacitance inside the oscillation circuit to thereby output the oscillation signal having temperature characteristics of low temperature dependency, a similar technology to which is also disclosed in JP-A-2007-208584 (Document 3).
In such temperature-compensated oscillator, there is a problem of reducing the power consumption while keeping the accurate temperature compensation. In Document 1 and JP-A-2-141026 (Document 4), there is disclosed a technology of decreasing the interval between the timings of generating the temperature compensation voltage when the temperature variation is great and increasing it when the temperature variation is small by contraries, and in Document 3, there is adopted a configuration of driving the temperature compensation circuit in the case in which the frequency of the oscillation signal runs off a certain allowable range centered on the reference frequency.
However, in the temperature-compensated crystal oscillator of Documents 1 and 3, there are a problem that the frequency changes rapidly due to the change in the capacitance since the change in the capacitance is discrete, and a problem that the cost is too high since it is required to increase the number of capacitors in order to improve the accuracy of the temperature compensation.
Further, since the temperature-compensated crystal oscillator of Document 2 monitors the actual frequency variation to thereby determine whether or not the temperature compensation is to be performed, there is a problem that the configuration for detecting the frequency becomes necessary, which increases the cost.
Further, in Documents 1, 3, and 4, although reduction in power consumption is achieved by changing the period of the timing of outputting the temperature compensation voltage, there is a problem that there is a limitation in reduction in power consumption since the temperature compensation circuit is driven continuously.